Houston, the most populous city in the Southern United States, is located along the upper Texas Gulf Coast, approximately northwest of the Gulf of Mexico at Galveston. The city, which is the ninth-largest in the United States by area, covers , of which , or 96.3%, is land and , or 3.7%, is water. Houston is located in the Gulf Coastal Plain biome, and its vegetation is classified as temperate grassland. Much of the city was built on marshes, forested land, swamp, or prairie, all of which can still be seen in surrounding areas. The city's topography is very flat, making flooding a recurring problem for its residents. The city stands about above sea level—the highest area within city limits being . However, subsidence, caused by extensive groundwater pumping and resource extraction, has caused the elevation to drop or more in certain areas. As a result, the city turned to surface water sources for its municipal supply, creating reservoirs such as Lake Houston and Lake Conroe (of which Houston owns two-thirds interest). Houston has four major bayous passing through the city: Buffalo Bayou, which runs into downtown and the Houston Ship Channel; and three of its tributaries: Brays Bayou, which runs along the Texas Medical Center; White Oak Bayou, which runs through the Heights and near the northwest area; and Sims Bayou, which runs through the south of Houston and downtown Houston. The ship channel goes past Galveston and into the Gulf of Mexico. Houston is located east of Austin, west of the Louisiana border, and south of Dallas. Houston County, Texas, located north of Houston, is unrelated to Houston. Geology Houston is largely located within the Northern Humid Gulf Coast Prairies level IV ecoregion (34a), a subset of the Western Gulf coastal grasslands level III region as defined by the United States Environmental Protection Agency. This region is generally underlaid by Quaternary-aged sand, silts, and clays (clay, clay loam, or sandy clay loam) and covered by grasslands with occasional clusters of oak trees. The land surface in and around the city of Houston is composed of alternating layers of red, gray, sandy brown, and black organic clay; these strata generally dip to the southeast at a slope of 0.06% ( of vertical change for every of distance traveled). These soils were deposited by tributaries of local waterways, particularly the Brazos and Trinity rivers. There is a considerable contrast in